Combat aircraft have always been vulnerable to fuel tank explosions caused by projectiles from enemy aircraft and ground artillery penetrating the fuel tank. A number of diverse approaches have been attempted to protect fuel tanks of combat aircraft from ballistic strikes. One technique has been to shield the tanks with protective armor to prevent ballistic projectiles from penetrating the tanks. However, this approach is not very well suited to aircraft applications where weight is an important consideration. Other approaches attempt to minimize damage once the projectile has penetrated the fuel tanks. For example, attempts have been made to prevent the fuel from exploding by fire-extinguishing systems that flood the tank with either inert gas, such as nitrogen or a fireextinguishing foam. These approaches are not totally effective, and they add undesirable weight to the aircraft. Fuel tanks have also been lined with rubberized membranes to seal the tanks against fuel leakage after being penetrated. However, this approach does nothing to prevent the fuel from exploding, and it is only effective for relatively small punctures.
Although fuel tanks can be penetrated by projectiles, the resulting entry holes in the tanks are relatively small. However, the projectiles can produce substantially larger holes and tears by a phenomena known as the "hydraulic ram effect." A high-speed projectile penetrating a fuel tank has a large amount of kinetic energy. As the projectile passes through the fuel in the tank, it is slowed by the fuel and its kinetic energy is transferred to the fuel as a pressure wave. The pressure wave then strikes the walls of the fuel tank over a relatively large area and can therefore create a relatively large hole or tear in the wall of the tank. The above-described approaches to protecting fuel tanks from ballistic projectiles do not address the hydraulic ram effect.
Protection systems that line the tank with armor or a sealing membrane are more vulnerable to the hydraulic ram effect where the tank must be penetrated by conduits such as fuel and vent lines. The tank must be sealed around the conduits to prevent leakage, and the high-intensity pressure wave produced by the hydraulic ram effect can rupture the seal and start a tear in the tank. It is also expensive and time-consuming to extend conduits through the tank and then seal the tank around the conduits.
Although the above-described problems may be most acute in fuel tanks for combat aircraft, similar problems exist for any vehicle, such as trucks, tanks, boats, etc., having a fuel tank that is subject to puncture by ballistic projectiles.